The present invention relates to solid state lasers having an output in the near infrared, especially those lasers which have a compact size and have a high peak output power.
Recent advances in lasers have led to monoblock lasers which feature a number of optical elements assembled on a substrate to provide a single piece laser of a compact size. These monoblock lasers are used in the area of range finding and surveying applications, and have been used in commercial and military applications. For example, U.S. Pat. Nos. 6,556,614, 6,373,865 and 6,744,801 to Nettleton show monolithic and pseudo-monolithic laser resonators on a substrate.
Monoblock laser resonators have a set of gain rods, generally a Q-switch and an OPO with various mirrors arranged linearly along the length of a substrate. (Monoblock lasers, as defined herein, have at least one optical resonator subassembly which is constructed by permanently and immovably affixing the individual optical components to a common substrate, usually by adhesive bonding, so that the subassembly is a unified, optically aligned, non-adjustable single article of manufacture.) Light from flash lamps or from diode lasers is coupled into a rod of lasing material such as Nd:YAG or the like, and the light moves bidirectionally through the rod, bouncing between a highly reflective (HR) mirror and an output coupler (OC) mirror placed at either end of the lasing rod. In range finding applications there generally is a Q-switch to provide a pulsed output. In many applications an eye-safe (near infrared wavelength (e.g., 1.54 micron)) laser output is desirable. To that end, an optical parametric oscillator (OPO) is introduced into the optical path, to change the output wavelength from 1.1 micron, the output wavelength of the commonly used Nd:YAG lasing material, to a desired eye-safe wavelength such as 1.54 micron.
Existing monoblock lasers use a UV sensitive adhesive to set the optical elements in place along an optical axis. The adhesive is placed between the optical elements and the substrate and the optical elements are located in appropriate locations on the substrate. Certain of the optical elements may be affixed to the substrate in a pre-alignment setting step. A light source is coupled to the resonator and while the monoblock laser operates, an intensity or a divergence of the output light is monitored. Certain critical elements, such as the HR mirror or the OC, are tilted in pitch and/or yaw, until the intensity or divergence meets a pre-set specification. Once the specification is met, UV light is applied to those optical elements not already fixed in place. Some designs feature optical elements with a rectangular cross section, placed on a flat substrate and other designs have optical elements with a round cross section, designed for a concave substrate. Regardless of the shape of the cross section of the optical elements, the movement necessary to the alignment process created unavoidably tapered glue joints between the elements and the substrate. The adhesive shrinks volumetrically during the curing process, so that thicker adhesive layers shrink more than areas where the adhesive is thinner. Uneven adhesive shrinkage pulls the optical elements out of their alignment with respect to the common optical axis depending on the geometry of the elements and the differences in thicknesses of the adhesive layer. For example, in an undesirable tapered glue joint between a mirror and the substrate (both with rectangular cross section), the thicker end of the glue contracts more than the thinner end, and the difference in shrinkage tilts a previously-aligned mirror out of alignment. This problem invariably degrades laser performance or causes outright failure, reducing manufacturing yield by increasing required rework.
Non-monoblock lasers address mirror misalignment with adjustable mechanical optic mounts which permit tipping the mirrors in pitch and/or yaw, or alternatively with mechanical mounts which permit rotational adjustment of one or more alignable Risley wedge pairs. (Risley wedge pairs permit changing the direction of light passing through the Risley wedge pair by rotating one wedge of the pair against the other.) The mechanical mounts may be equipped with locking means (e.g., clamps, screws and the like) to secure the mechanical alignments, once alignment is achieved. Mechanical mounts generally add cost, size and weight to the non-monoblock laser and consist of several moving parts which introduce additional possibilities for subsequent misalignment.
Both problems degrade laser performance and reduce manufacturing yields by increasing re-work and failures, as well as requiring more power to operate a less than optimally aligned laser.
Consequently, a need exists for a more efficiently manufacturable laser, especially a monoblock laser, which substantially reduces misalignment of optical elements due to adhesive shrinkage.